a lot of what you are seeing, I believe, is due to the quality of the meats you are cooking, you said it yourself that on two occasions one meat was better marbled than the other and the better marbled one turned out better...
next time cook at 275*, foil the meat at 160* internal with some apple juice added and finish the cook until the meat is probe tender....probably around 200* internal...
and remember, it's just barbecue, have fun with it!!!